In the operation of machine tools, and in particular presses, clamping devices serve for clamping workpieces and/or tools, for example to the machine table, press ram or plate holder. The clamping devices are conventionally actuated hydromechanically, the actual clamping force being generated for example by a mechanical toggle lever system.
A hydromechanical quick-acting clamping device which is disclosed in German Patent Specification No. 2 233 940 has a fluid pressurised piston, which is slidable transversely to the clamping direction in a housing, and at least one clamping lever, which bears at one end against the piston and at the other end against a pressure plate. The plate projects from the housing in the direction of clamping and is provided with a clamping anchor. The lever is pivotal about its end nearest the piston between at least one inclined position and one extended position in respect of the clamping direction. The pressure plate is fluid-pressure-connected at its piston-side boundary surface with a pressure chamber of the piston which causes clamping and is fluid-sealed with respect to the housing. In this way, high clamping forces and the least possible axial play of the clamping anchor are ensured; that is to say there are relatively large idling strokes within the very rapid clamping and release cycle. A functional division between the initial idling stroke and the succeeding, controlled clamping and locking of the clamping anchor can be achieved.
In order, in the case of clamping devices in which the clamping forces are produced by a hydromechanical toggle lever system, which permits only small clamping strokes when large forces are generated, to be independent of the necessarily tight thickness tolerances that must be observed, a compensating unit may be provided for the infinitely adjustable coverage of clamping movements up to approximately 15 mm. Such units are suitable especially as a supplementary device for hydromechanical clamping devices, and such an arrangement is disclosed in German Gefrauchsmiester Specification No. 8 204 128. This compensating unit comprises, substantially in an undivided housing, two coaxial screw-threaded bolts cooperating with internal threads of nut-like housing components. One of the bolts has a lefthand and the other a righthand thread, and the bolts are adjustable by means of a hydraulic rotary drive, to enable the housing components to be axially separated one from another.